1. Field of the Invention
The present invention relates to a method of manufacturing a multilayer wiring substrate in which two or more wiring substrates are stacked on each other, and a multilayer wiring substrate.
2. Description of Related Art
In association with the miniaturization and the usage of multiple functions in electronic apparatuses, the request for higher density of wiring substrates (printed circuit boards) and miniaturization of mounted parts are becoming increasingly restrictive. In the wiring substrate, the higher density in a direction parallel to a substrate surface has been conventionally tried by reducing a wiring rule. However, in recent years, the higher density in a direction vertical to the surface of the wiring substrate has been advanced by employing a buildup process, stacking the wiring substrates, and forming via holes (interlayer connections) to electrically connect any layers to each other.
As this type of a conventional technique, for example, Japanese Laid Open Patent Application (JP-A-Heisei, 10-107445) discloses a method of manufacturing a multilayer wiring substrate, which forms a conductor pattern (a wiring pattern) on a wiring substrate by using a transferring method. The conventional method of manufacturing a multilayer wiring substrate will be described below with reference to FIGS. 15A to 15E.
At first, as shown in FIG. 15A, a wiring substrate in which a first conductor pattern 2 is formed on a surface of an insulator 1 is produced or prepared. Then, as shown in FIG. 15B, an insulating layer 3 is formed by coating insulation slurry on the surface of the insulator 1. Next, as shown in FIG. 15C, via holes 4 linked to the first conductor pattern 2 are formed in the insulating layer 3 by using laser machining and the like, and a conductive paste 5 is filled in the formed via holes 4. In succession, as shown in FIG. 15D, a second conductor pattern 6 previously formed on a transfer sheet 7 is transferred onto the insulating layer 3. The first and second conductor patterns 2, 6 are connected through the conductive paste 5.
The transfer sheet 7 is mainly made of synthetic resin material such as polyethylene terephthalate (PET) and the like. The conductor pattern 6 is formed by patterning a conductor layer put or deposited on this transfer sheet 7 to a predetermined shape by using a wet etching method. The transfer of the conductor pattern 6 to the insulating layer 3 from the transfer sheet 7 is carried out by using the differences in adhesion between the conductor pattern 6 and the insulating layer 3 and between the conductor pattern 6 and the transfer sheet 7.
When three or more conductive layers are formed, the processes similar to the above-mentioned case are repeated. In short, as shown in FIG. 15E, an insulating layer 8 is further formed on the insulating layer 3. Via holes are formed in this insulating layer 8, and a conductive paste 10 is filled in the formed via holes. After that, a third conductor pattern 11 is formed by the transferring method.
As mentioned above, in the case of the multilayer wiring substrate manufactured by the transferring method, the via holes for the connection between the layers are formed in any portions of the insulating layer. Consequently, the multilayer can be easily attained.
However, in the case of the above-mentioned conventional method of manufacturing the multilayer wiring substrate, the transfer sheet 7 is mainly made of the resin film. Thus, this method has a problem that the expansion/contraction and the warp of the transfer sheet 7 occurring at a time of handling cause an error to be easily induced in the pattern shapes (the dimensions) of the transferred conductor patterns 6, 11. Thus, in this conventional method of manufacturing the multilayer wiring substrate, it will be very difficult to cope with the hyperfine structure (fine pitch structure) of the conductor patterns expected to continue to evolve in the future. Hence, it is impossible to obtain the multilayer wiring substrate of a high quality which corresponds to the fine pitch structure.
By the way, the transfer sheet is thought to be made of metal material such as stainless steel and the like. In this case, as compared with the case in which the transfer sheet is made of the resin film, a rigidity of the transfer sheet is increased to thereby improve the dimensional stability of the conductor pattern. However, in this case, if the rigid property of the transfer destination is strong, it is difficult to remove the transfer sheet. Thus, this method has a problem that the operation for transferring the conductor pattern is not able to be properly done.
Also, in the case of the conventional method of manufacturing the multilayer wiring substrate, the process for producing the multilayer is the steps of alternately stacking the insulating layer and the conductive layer by one layer at a time. For example, if any step defect occurs in the upper layer, all of the steps until that time become vain, and the entire wiring substrate is treated as a defect. Thus, the conventional method of manufacturing the multilayer wiring substrate has a problem that its productivity is poor and its yield is low.
Moreover, the conventional method of manufacturing the multilayer wiring substrate is designed so as to form the insulating layers 3, 8 on the entire surface of a bedding layer when producing the multilayer. Thus formed insulating layers 3, 8 need to be baked and cured. Thus, the conventional method must impose a certain limit on the selection for the construction materials of the insulating layers 3, 8, in order to protect the miss match of a baking temperature. Consequently, this method has a problem that the degree of the freedom of the board design becomes low.
On the other hand, when the board design is carried out for producing the multilayer only in a partial region on the bedding substrate and improving the wiring density, the conventional manufacturing process needs to uniformly form the insulating layers even in the other regions on the bedding substrate. Thus, the conventional method also has a problem that the burden of a material cost when an expensive material is used for the insulating layer is increased.
As mentioned above, the conventional method of manufacturing the multilayer wiring substrate has the problems that it is difficult to cope with the finer pitch structure of the conductor pattern and that the restriction on the material selection is always accompanied and that the producing cost or the material cost is expensive.